Vacuum processing systems for processing 100 mm, 200 mm, 300 mm or other diameter substrates are generally known. Typically, such vacuum processing systems have a centralized transfer chamber mounted on a monolith platform. The transfer chamber is the center of activity for the movement of substrates being processed in the system. One or more process chambers attach to the transfer chamber for performing one or more of a variety of different processes on the substrates. Access to the transfer chamber for substrates from the exterior of the system, or from the manufacturing facility, is typically through one or more load lock chambers. The load lock chambers cycle between the pressure level of the ambient environment and the pressure level in the transfer chamber in order for the substrates to be passed therebetween. The load lock chambers attach to a substrate transfer chamber, referred to herein as a mini-environment, which handles, or transfers, substrates in a very clean environment at atmospheric pressure from substrate pods to the load lock chambers. The interiors of each of these chambers must be maintained very clean so as not to contaminate or damage the substrates that pass therethrough.
FIGS. 2 and 3 generally show a vacuum processing system 100 having a mini-environment 120 with a clean-air supply system 200 disposed on the top of the mini-environment 120. The mini-environment 120 has a substrate handler 128 that transfers substrates 156 from substrate cassettes 154 supported on load ports 122 to load lock chambers 118. If airborne particles, or contaminants, deposit on the substrates during handling in the mini-environment, then the integrity of devices formed by the processes that the substrates are to undergo may be adversely affected. In order to maintain the cleanliness of the mini-environment, highly filtered air is continuously flowed therethrough. A clean-air supply system, typically disposed on the top of the mini-environment, has a scroll-type centrifugal fan assembly generating airflow through a high efficiency particle filter. The filtered air flows downward to remove, by airflow and gravity, any airborne particles that may have entered the mini-environment or have been shed from internal surfaces or devices.
The prior art for the clean-air supply system 200 uses a single centrifugal fan assembly 10, such as that shown in the side and top views of FIGS. 1a and 1b, respectively, to flow highly filtered air generally downward in the direction of arrows F-H. The filtered air flows out through exit vents 202 arranged in the bottom of the mini-environment 120.
The centrifugal fan assembly 10 has a motor 12 mounted on supports 14 with a drive shaft 16 connected to a squirrel-cage fan 18 to rotate the fan 18. The fan 18 mounts inside a fan housing 20 and rotates in the direction of arrow D to generate airflow through to an inlet plenum 22 and into a filter plenum 24 where the airflow is spread out and redirected downward through a high-efficiency particle filter 26 and into the mini-environment 120. A continuous flow of air by the fan assembly 10 ensures that the air in the mini-environment 120 will be maintained in a very clean condition.
A problem with the prior art clean-air supply systems 10 is that centrifugal fans do not provide an efficient means to control the speed of the airflow through the mini-environment 120. It is necessary to vary the airflow rate in the mini-environment 120 at various times in the operation of the processing system 100. Occasionally, the airflow rate must be increased to more quickly remove particles from the mini-environment, such as at start-up of a new system 100 since a relatively large number of particles may have accumulated in the mini-environment 120 by that time, or to prevent particles from entering the mini-environment, such as when opening or closing load port doors 123 so contaminated air from outside the mini-environment 120 or particles generated by the movement of the doors 123 do not enter the mini-environment 120. Thus, a high airflow rate is needed for rapid clean-up of the mini-environment 120, but it is also used to test the integrity of the seals of the mini-environment 120, by providing positive pressure inside the mini-environment 120 which may cause a detectable air-jet at a leakage point in any of the seals, and to measure the air pressure drop across the filter, whereby an unacceptably high pressure drop indicates that the filter has collected too many entrained particles to continue functioning properly and needs to be replaced. A high airflow rate, however, means a high fan speed, which causes unwanted noise in the manufacturing facility, draws significantly more electrical power than at slower speeds and shortens the useful life of the filter. It is desirable, therefore, to slow the fan speed down whenever possible to reduce noise, conserve energy and lengthen the life of the filter. Thus, when no substrates are being passed through the mini-environment 120 or the system 100 is otherwise in a standby mode, or waiting, it is preferable to reduce the fan speed and the airflow rate. The fan assembly 10 is rarely turned completely off, however, since the number of particles may build up inside the mini-environment 120 while the air therein is still, so the fan assembly 10 is typically maintained at least at a minimal speed. Additionally, the fan assembly 10 may be operated at an intermediate level at times, such as when substrates are being moved through the mini-environment 120, and it is not necessary to flow the air very fast, but it is desired to maintain the level of cleanliness fairly high in the mini-environment 120.
One prior art means to control the airflow speed has been to vary the axial speed of the centrifugal fan 18. The most often used means to vary the fan speed has been to incorporate an electronic variable speed fan control unit. Reducing the axial speed of the fan 18, even though the power consumed by the fan assembly 10 may be less, can still result in power losses, since the fan assembly 18 is typically rated to operate at a particular power level, the fan assembly 18 inefficiently uses the available power at lower power levels. An additional issue with using variable speed fan control units is that for the relatively small units typically needed in a mini-environment, the electronics have electrical noise and heat dissipation issues that do not meet European CE EMC requirements. These units typically moot U.S. standards, but not European requirements. Thus, although a manufacturer of processing systems 100 and/or mini-environments 120 may prefer to build one type of system 100 with only one mini-environment 120 using only one type of clean-air supply system 200 for all customers in all countries, the standards requirements of some countries makes such a preference impossible or costly.
Another prior art means to control the airflow speed has been to partially close off the airflow through the fan assembly 10 by means of a damper. In this manner, the fan assembly 10 always operates at the same speed, but the damper partially blocks the airflow passageway into or out of the fan assembly 10. A damper in the airflow, however, merely causes the fan assembly 10 to generate airflow against the damper and lose part of the air pressure that it generates. Thus, a damped clean-air supply system 200 wastes energy, due to the intentional loss of airflow.
Therefore, a need exists for a processing system with a mini-environment and a clean-air supply system that can vary the airflow through the mini-environment while efficiently using the power available to its airflow mechanism and easily meeting world market standards for electrical noise and thermal dissipation.